1. Field of the Invention
This invention relates generally to thermal exposure monitors, and more particularly to a cumulative thermal exposure monitor having a housing with a fluid sealed interior cavity and a thermally-responsive member therein formed of a shape memory material that moves an indicator wherein the thermal properties of the housing, cavity, and shape memory material transition temperature range are calibrated relative to one another to function in mutual cooperation such that the thermally-responsive member gradually changes from a first shape to a second shape upon absorption of heat energy over time to closely match a thermal deterioration profile of a perishable product, and to indicate the cumulative amount of heat energy absorbed and whether, at any time, the temperature has been above a preferred storage temperature range for a period of time sufficient to cause any degree of deterioration based on the known time and temperature thermal deterioration profile of the perishable product.
2. Brief Description of the Prior Art
It is of utmost importance to maintain processed produce such as food products and perishable non-food products such as medical products, drugs, research reagents, pharmaceuticals, human organs and tissues, etc., within a predetermined temperature range and to determine whether such products may have been exposed to detrimental temperatures outside of a specified temperature range during some step in the chain of distribution from the source to the destination.
The demand for processed produce and perishable non-food products has pushed the limits of the distribution systems of the perishable products industry to maintain an unbroken temperature controlled chain from field to consumer and industry. Many segments of the food industry utilize a Hazard Analysis and Critical Control Point system (HACCP), which is a world-recognized, common sense approach to food safety and prevention of food contamination.
The HACCP has outlined seven principles for food safety and prevention of food contamination: (1) Conduct a hazard analysis to identify potential hazards that could occur in the food production process; (2) Identify the critical control points (CCPs)—those points in the process where the potential hazards could occur and can be prevented and/or controlled; (3) Establish critical limits for preventive measures associated with each CCP; (4) Monitor each CCP to ensure it stays within the limits; (5) Take corrective actions when monitoring determines a CCP is not within the established limits; (6) Keep records that document the HACCP system is monitored and working correctly; and (7) Verify that the HACCP system is working properly through tests and other measures.
Existing HACCP programs in place at the processing end of the chain are not equipped with proper tools to monitor all of the critical control points (CCP's) in the cold chain, and temperature related problems can still occur throughout the distribution chain. Thus, there is a need for a device that can be used for economically monitoring the links in the cold chain, especially when perishable loads are broken up and re-distributed.
Temperature monitors and indicators are known in the art. There are several patents that disclose various temperature indicating devices.
Bradbury, U.S. Pat. No. 2,966,281 teaches a “snap-action” temperature-sensing device having a thermally responsive bimetallic inverted V-shaped spring (not shape memory alloy or polymer material), and a latch insertable into the device to temporarily hold the spring member in an initial position. The apex of the V-shaped spring serves as a pointer and indicia on the housing represent zones corresponding to the location of the apex to indicate the temperature of the monitored product.
Rogen et al U.S. Pat. No. 3,483,752 discloses a temperature monitor using a shape-memory alloy sensor disposed in a compartment in a housing preferably constructed of transparent acrylic plastic that has: (1) a thin wall section that is affixed onto the package of a medium being monitored to serve as a preferential heat conducting path between the monitored medium and the sensor is affixed onto the package of a medium being monitored, and (2) a thick wall section that serves as a preferential insulating path between the sensor and the ambient environment. The sensor must respond decisively (instantaneously) to a small temperature change and can be made to actuate (change shape) more rapidly than the monitored medium, to insure that the monitored medium (blood) is either safe or unsafe, but otherwise remains dormant. This device does not utilize a scale because there is no degree of spoilage of blood (it is either considered good or bad). It also requires the user to observe the configuration of the shape memory alloy sensor (whether it is straight or coiled, curled or flat, or twisted or flat) in order to determine whether the monitored medium has exceeded the specified temperature. This device differs from the present invention in that it is strictly a binary device (it can only give you a yes or no answer), it is designed to measure whether a temperature threshold has been exceeded, and its housing merely isolates the mechanism from its environment and links it to the medium.
Dewaegheneire, U.S. Pat. No. 4,448,147; Weynant nee Girones, U.S. Pat. No. 5,018,874; and Darringer et al, U.S. Pat. No. 5,076,197 disclose “step” type temperature monitors. These devices differ from the present invention in that they merely measure and indicate whether one or more temperature thresholds have been achieved.
Bianchini, U.S. Pat. No. 5,531,180 teaches a device utilizing a pre-loaded tension spring or leaf spring (not shape memory alloy or polymer) in cooperation with a fluid material having a controlled temperature dependent viscosity, wherein the viscosity changes as a function of temperature. The spring and fluid are located in a compartment in a transparent housing or case that is affixed onto the package of a medium being monitored. The end of the spring has an enlargement or indicator in the form of a small sphere or protuberance, which is held in a first position by a removable retaining pin. After the device along with the monitored medium has been frozen, the retaining pin is removed. As a result of the viscosity of the temperature dependent fluid or material, with the elapsing of time, the pressure of the spring will cause the indicator to move to a second position to indicate that the frozen product has gone through “thermal mishandling” in is unfit for consumption. The time required for the indicator to move to the second position is proportional to the viscosity and to the radius of the fluid friction surface (exterior surface of the spherical indicator) and inversely proportional to the elastic constant of the spring, which urges the indicator through the viscous medium, or (with the protuberance) the area perpendicular to the motion of the indicator toward the second position. This device operates on entirely different principles than the present invention and relies on the relationship of viscosity of a fluid with respect to temperature.
Shahinpoor et al, U.S. Pat. No. 5,735,607 discloses a temperature sensor having an indication surface, at least one shape memory alloy (SMA) member with a first shape at temperatures below a critical temperature and a second shape at temperatures above the critical temperature, and a plurality of indicators mounted with the members which obscure the indication surface when the members are in the first shape, and do not obscure the indication surface when the members are in their second shape. The shape change of the SMA element causes the sensor to change between two readily distinguishable states to indicate that a temperature threshold was exceeded, and must always be maintained at a temperature below the transformation temperature of the shape memory alloy member(s) until the beginning of the sensing operation.
Waynant nee Girones, U.S. Pat. No. 5,335,994 discloses a temperature monitoring device having a casing made of synthetic material that contains a motor element with it a movement transmission element consisting of a piston and rod and a shape memory alloy spring acting on at least one indicator element irreversibly to record each overstepping of a predetermined threshold temperature. The device is capable of having a variable response time (“delay time”) on each of the temperature thresholds. One embodiment of the device enables the durations of the overstepping of the various temperature thresholds to be indicated. The piston and rod are supported at one end of the shape memory spring in a chamber. An optional balancing spring bears against the piston. The chamber has a second part, separated from the first by a baffle which contains a clockwork mechanism coupled to a revolution-counter mechanism, the triggering of which is effected with the aid of pushers operating a switch. At ambient temperature the balancing spring is compressed between the baffle and the piston, the switch is not operated and the clockwork movement indicates a zero duration. When the temperature falls to the set temperature the spring contracts, allowing the piston to move back which pivots the pusher, which triggers the switch and the clockwork mechanism starts to operate to indicate the duration of the low temperature based on the number of revolutions of the revolution counter.
A “temperature sensitive” product does not decay or become spoiled as a result of exposure to a given temperature; instead, it spoils due to the amount of heat imparted to it as a result of a temperature difference over time. In other words, a given product, such as a food product or produce, can safely tolerate short exposures to an elevated temperature, but not long exposures. Thus, prior art devices that merely indicate that a temperature threshold was achieved or exceeded at some point in time do not indicate the cumulative amount of heat energy absorbed within a preferred storage temperature range and whether, at any time, the cumulative heat energy absorbed has taken place for a period of time sufficient to cause any degree of deterioration based on the time and temperature thermal deterioration profile of the perishable product.
The present invention is distinguished over the prior art in general, and these patents in particular by a cumulative thermal exposure monitor for monitoring heat energy absorbed over time and indicating the degree of deterioration based on a time and temperature thermal deterioration profile of a perishable product that has a known time and temperature thermal deterioration profile and is prescribed to be maintained within a preferred storage temperature range. The device has a thermally-conductive housing with a fluid sealed interior cavity and a transparent window portion, the housing formed of a material having thermal capacity and insulative properties which moderate heat energy conducted therethrough over time. A thermally-responsive member movably disposed in the interior cavity formed of a shape memory material has a phase transition temperature range encompassing at least the preferred storage temperature range and has a first shape at temperatures below the phase transition temperature range and gradually changes to a second shape at temperatures within or above the preferred storage temperature range. An indicator visible through the window portion is moved by the thermally responsive member from an initial position as it gradually changes from its first shape to its second shape. The thermal capacity and insulative properties of the housing material and shape memory material phase transition temperature range are calibrated relative to one another to function in mutual cooperation such that the housing moderates heat energy conducted therethrough over time and the thermally-responsive member absorbs the heat energy and gradually assumes its second shape upon absorption of heat energy over time to closely match the known thermal deterioration characteristics of the perishable product. The indicator remains substantially in a position at which it was last moved by the thermally-responsive member, regardless of subsequent exposures of the shape memory material to lower temperatures, to indicate the cumulative amount of heat energy absorbed in and above the preferred storage temperature range and whether, at any time, the temperature has been above the preferred temperature range for a period of time sufficient to cause any degree of deterioration based on the time and temperature thermal deterioration profile of the perishable product.